A switched reluctance motor is an electrical motor that includes a rotor and a stator. Torque in a reluctance motor is produced by the tendency of the rotor to move to a position relative to the stator in which the reluctance of a magnetic circuit is minimized, i.e. a position in which the inductance of an energized stator winding is maximized. In a switched reluctance motor, circuitry is provided for detecting the angular position of the rotor and sequentially energizing phases of the stator windings as a function of rotor position.
Switched reluctance motors are doubly salient motors having poles on both the stator and the rotor, with windings only on the stator poles. The rotor of a switched reluctance motor does not include commutators, permanent magnets, or windings. Switched reluctance motors have a variety of uses, including vacuum cleaners, for example.
Torque may be produced by energizing or applying current to the stator windings of the stator poles associated with a particular phase in a predetermined sequence. The energization of the stator windings is typically synchronized with the rotational position of the rotor. A magnetic force of attraction results between the poles of the rotor and the energized stator poles associated with a particular phase, thereby causing the rotor poles to move into alignment with the energized stator poles.
In typical operation, each time a stator winding of the switched reluctance motor is energized, magnetic flux flows from the energized stator poles associated with a particular phase, across an air gap located between the stator poles and the rotor poles, and to the rotor poles. Magnetic flux generated across the air gap between the rotor poles and the stator poles produces a magnetic field in the air gap that causes the rotor poles to move into alignment with the energized stator poles associated with a particular phase, thereby producing torque. The amount of magnetic flux and, therefore, the amount of torque generated by the switched reluctance motor is dependent upon many variables such as, for example, the magnetic properties of the material of the rotor poles and the stator poles, and the length of the air gap between the rotor poles and the stator poles.
The magnetic flux generated can be divided into a main torque-producing flux and leakage flux. The main flux is the flux that flows through the rotor poles and the excited stator poles. This main flux produces a torque on the rotor that will tend to align the rotor poles through which the flux passes with the excited stator poles. Leakage flux is undesirable in switched reluctance motors because it directly reduces torque production. More specifically, leakage flux causes the motor to produce a torque in a direction that is opposite to the direction of rotation of the rotor, also known as a braking torque. It is known that modifications to the rotor pole face may affect torque production in the switched reluctance motor.
Conventional switched reluctance motors utilizing a micro-controller to control the commutation of power provided to the stator windings perform the same start-up routine whenever power to the circuit is turned on. However, if the power to the motor is turned off when the rotor is rotating at a high rate of speed and then quickly cycled back on, using the same start-up routine often causes damage to occur to the electrical components in the motor. Typically, it is the insulated gate bipolar transistors (IGBTs) in the circuit that are most susceptible to damage if the motor is not allowed to coast for a period of time until the rotational speed falls below a threshold speed.